This invention relates to a photosensitive glass which colors by exposure to ultraviolet rays and undergoes thermal bleaching, and which turns into an opaline foam glass by heat treatment at a temperature above the softening temperature.
A typical example of known photosensitive glasses is the so-called photochromic glass which is obtained by precipitation of fine particles of a noble metal or heavy metal halide in the matrix of a borosilicate glass. The manufacture of this photochromic glass requires advanced and sophisticated techniques and very troublesome operations and entails high costs. Another reason to limit application of this photochromic glass is that it is very difficult to produce wide sheets of this glass.
Another example of known means to obtain a photosensitive glass is doping of a silicate glass with a rare earth element. However, this method is hardly put into practical use because of significant fatigue and deterioration of the obtained photosensitive glass.
There are some proposals for photosensitive glasses using neither noble metals nor rare earth elements. For example, Japanese patent application primary publication No. 58-204839 (1983) shows a UV-sensitive glass which is composed essentially of SiO.sub.2, alkali metal oxide(s) and alkaline earth metal oxide(s) and may optionally include Al.sub.2 O.sub.3 and/or B.sub.2 O.sub.3. However, this photosensitive glass is not very suitable for practical use because both the rate of coloration by exposure to UV rays and the intensity of coloration are too low.
Changing the topic, there are various types of translucent or opaline glasses having the effect of scattering light. The most popular and primitive one is frosted glass produced by grinding one side of sheet glass. Frosted glass is inexpensive and is effective in preventing seethrough, but it lacks elegance from an aesthetic or ornamental point of view and has a disadvantage that the light scattering effect is lost, for example, by condensation of moisture on the ground glass surface. Also it is well known to produce a so-called milky glass or opal glass by adding an opacifying agent such as a phosphate or a fluoride to a silicate glass. In practice, however, it is not so easy to very uniformly disperse the opacifying agent to thereby surely prevent irregularity in opacification. Therefore, the production requires special facilities including stirring means provided to the melting apparatus, so that the cost of production is not satisfactorily low.
Japanese patent application primary publication No. 49-90709 (1974) shows an opaline glass not containing any opacifying agent. Essentially this glass is composed of SiO.sub.2, Al.sub.2 O.sub.3, CaO and MgO, and opalization is accomplished by performing melting of the batch mixture at a specific temperature so as to cause phase separation, lowering the temperature of the melt to a specific level at the stage of forming the glass into a sheet or otherwise and cooling the formed glass at a specified cooling rate. Although this opaline glass is low in the material costs, the manufacturing operations require complicated techniques for precisely controlling the temperatures and the cooling rate.